Measurement of cross-section geometry effects on electron collection to long probes in mesosonic flowing plasmas

This paper presents the analysis and comparison of measured electron current collection to cylindrical, solid tape, and slotted tape electrodynamic-tether samples in a mesosonic flowing plasma. A Hall thruster was used to simulate a flowing unmagnetized space plasma in a large 6m× 9m vacuum chamber. Guarded tether samples were designed to mitigate end effects. Plasma parameters were determined based on the ion saturation and electron retardation regimes of a Langmuir probe's current characteristics. Solid tape samples with effective widths spanning from 4.9 to 41.9 Debye lengths, and slotted tapes with line spacings spanning from 1.4 to 13.2 Debye lengths were tested. Several conclusions can be drawn from the analysis of the results: 1) The plasma flow leads to current enhancements over that predicted by the orbital-motion-limited theory; 2) the electron collection efficiency of solid tapes (on a per area basis) decreases as the width of the tape is increased; 3) beyond a threshold bias close to the beam energy, solid and slotted tapes both collect more current when oriented transverse to the flow; 4) equivalent width slotted tapes are more efficient electron collectors than solid tapes on a per area basis; 5) our data suggests the electron collection efficiency of slotted tapes decreases with increasing line spacing until a possible minimum efficiency is attained, beyond which it is expected to start increasing again. The minimum was attained in the case of the samples oriented transverse to the flow, but not in the case of the samples aligned with the flow, for which the critical spacing is likely higher due to an increased sheath interaction radius of each line caused by the elongation of the sheath associated with plasma flow.